Workers exposed to irritating dusts and fumes have been known to exhibit changes in lung function throughout a work shift to which they are assigned. Standard spirometry data collected before and after the work shift may detect a change in lung function from the beginning to the end of a work shift. However, it is more important to track changes in lung function and capacity on a substantially real-time, periodic basis, whether throughout the work shift or over the course of an entire day (or days).
Similarly, asthmatics may experience randomly occurring attacks. While lung function is adversely affected during the attack, lung function may return to normal thereafter. For these individuals, spirometric data collected during an attack is a more accurate representation of the nature of the attack and is thus more desirable to obtain than data indicative of lung function only at the beginning and end of an attack.
Portable, or hand held, prior art spirometric devices are commercially available. Such devices are known in the art. The known devices operate as follows.
Volume spirometers provide the simplest approach to measurement of spirometric parameters. Volume spirometers are essentially large cylindrical chambers including displaceable pistons. A test subject forces air into the chamber by performing a forced vital capacity (FVC) maneuver. The piston displacement corresponds to the volume of air being expired. Although accurate, these devices are large and bulky.
To keep the size and weight of a spirometric device to a minimum, it is known to measure flow instead of volume. Thus, in a flow-spirometer a flow sensor outputs a flow signal indicative of detected air flow thereacross. The desired volume data is then determined by mathematical integration of the flow signal. An advantage of such an approach is that flow spirometers are inherently smaller than volume spirometers. However, although implementable by a smaller device, the flow-sensing method of obtaining spirometric data is known to be less accurate and more sensitive to errors than the volume based method.
The volume of gas exhaled into both volume and flow-based spirometric devices is initially at 37 degrees C. and rapidly cools to ambient temperature. This cooling to ATPS (Atmospheric Temperature Pressure Saturated) causes a contraction of the gas from the volume occupied at BTPS (Body Temperature Pressure Saturated) in the subject's lungs. The spirometric volume measurement must therefore be multiplied by a BTPS correction factor to obtain the volume value at body temperature.
Portable flow type spirometric systems fall into one of two categories: (1) peak flow meters and (2) pneumotach systems.
Peak flow meters are very simple mechanical devices consisting of a mouthpiece and an indicator gauge. When the subject performs a FVC maneuver, the force of the expired air moves an indicating marker along a calibrated dial allowing peak flow to be read. If such a device is used without the aid of an administering technician, it is the subject's responsibility to perform the maneuver with sufficient effort, read the graduated scale correctly, and record the value along with the time of day. Peak flow is the only information that can be obtained from this type of device.
Portable pneumotach systems comprise a flow sensing pneumotach which generates an electrical flow signal proportional to flow. The flow signal is sampled periodically by a microprocessor, which then evaluates and stores the data.
However, because of lack of resolution in the electronic analog-to-digital converter (ADC) device which converts the analog flow signal to digital form usable by the microprocessor, there result deficiencies in accuracy of the measurement. This difficulty manifests itself most prominently at low flow rates as a lack of flow resolution and results in significant volume errors when the flow is integrated over time to obtain volume.
Similarly, because the pressures being measured are relatively small (less than 1 inch of water at 12 liters/second) and solid state transducers functioning adequately in this range suffer from large offset drifts due to temperature, accuracy is still further reduced. In a known device, the pressure transducer has an output of 2.5 mv per inch of water and a temperature offset drift of typically plus/minus 3 mv from 0 to 25 degrees C. In the known device, no drift compensation is provided. Instead, the transducer output span is reduced to allow for the large fluctuation in offset drift. However, this drift can be greater than the output span.
The processing circuitry used in such devices further results in flow errors caused by a lack of resolution therein. Although individual flow errors are quite small (a maximum of 1 part in 4096 for an existing 12 bit ADC), when integrated over extended periods of time to determine volume, the resulting volume errors can become significant. However, resolving the problem by using higher resolution electronic ADC's is expensive, and requires significant additional electronic circuitry.
The flexibility of these types of portable pneumotach systems is further limited by the microprocessor software, which is usually stored in a read-only-memory (ROM). It is known that different sampling programs may be needed, or provided, for different spirometric units, depending on the specific application contemplated. However, for known spirometric devices the sampling program is part of the software stored in read-only-memory (ROM). Thus, modification of the sampling (as well as other) software is not possible without disassembling the device and replacing the ROM chip.
Further, known devices fail to provide for dynamic computation of the body-temperature-pressure-saturated (BTPS) correction factor (CF). That is, where non-heated ceramic flow sensors are used, typically there is only incomplete cooling of the flowing air as the air passes through the sensor. Thus, the usual technique applies a factor approximately equal to thirty percent (30%) of the full BTPS CF. Upon testing of several ceramic flow sensors with a mechanical pump, using both room air and air heated to 37.degree. C. and saturated with water vapor, the inventors discovered the following.
Upon using volume ramps and the first four ATS standard waveforms to test the sensors, and upon calculating an estimated BTPS correction for FVC and forced expiratory volume in 1 second (FEV1) by dividing the volume measured with room air by the volume measured with heated and humidified air, the results using room air showed considerable variability in the linearity of the flow sensors. One sensor showed a 400 ml difference (6.7%) in a 6 L volume ramp and flow rates of between 0.6 and 8 L/s. Using heated and humidified air, the estimated BTPS CF with the sensor initially at 20.degree. C. ranged from 1.06 to 1.00, compared to a calculated value of 1.102. The estimated BTPS CF also varied with the number of curves previously performed, the time between curves, the volume of the current and previous curves, and the temperature of the sensor.
Thus, the known devices suffer from inaccuracies occasioned by the known technique of using ambient temperature to estimate the temperature of the flow sensor, as well as from errors arising from use of a single, static, BTPS correction factor for all parameters, without regard to the exact time that specific measurements are made during a forced exhalation and to time related temperature variation.
Additionally, known devices convert the raw spirometric data to various parameters descriptive of the user's lung capacity, storing only the resultant parameters rather than the raw data. Thus, known devices lose any capability to perform further computations on the raw data and to abstract still further information therefrom.
Moreover, known devices do not include a provision for reminding a user to obtain data periodically.
Still further, known devices suffer from inaccuracies caused by flow sensor nonlinearity and flow sensor drift.
All of the commercially available portable flow devices known to the inventors thus suffer from limitations in accuracy, flexibility, data storage capacity and physical size, and lack specific desirable options and features. The prior art devices are thus inadequate for remote and prolonged data collection.
There is accordingly a need in the prior art for a portable spirometric device capable of providing spirometric data having improved accuracy and reliability and providing an improved resolution in the conversion process.
There is a more specific need in the prior art for a flow type spirometer including a capability for dynamic computation of the BTPS correction factor.
Still another need of the prior art is for an ability to provide actual sensor temperature values for the flow sensor of a portable spirometer.
There is a further need for a portable spirometric device having a capability for accepting different operating control programs for flexible adaptivity to various applications, including accepting differing sampling programs, without requiring disassembly or replacement of a ROM therein.
There is yet another need in the prior art, for spirometric devices including means for correcting inaccuracies caused by flow sensor nonlinearity and flow sensor drift.
Additionally, there is a need in the prior art for a portable spirometric device having improved storage capacity for raw spirometric data, to enable subsequent processing thereof.
There is moreover a need in the prior art for a portable spirometric device which periodically reminds its user to perform a FVC maneuver to obtain periodic spirometric data.
There is still a more specific need in the prior art to provide increased resolution in the analog to digital conversion process without incurring the additional costs associated with higher resolution ADC's, thus to permit the use of lower resolution ADC's with minimal additional circuitry and oversampling to improve the existing resolution capability thereof.
There is yet another need in the prior art to provide temperature drift compensation for the transducers used in known devices, thus to increase the effective transducer span and to extend the operating temperature range of the device.
Further, there is a need in the prior art for a portable spirometric device having a reduced size to assure that a subject will not be discouraged by bulkiness of the device from carrying the device and using the spirometer as required.